The invention relates to an eddy-current brake device of the type comprising at least one heat exchanger for dissipating the thermal energy that is due to the eddy currents during braking.
To measure engine performance, particularly the performance of internal combustion engines, use is made of braking devices connected to the output shaft of the combustion engine.
In eddy-current brake devices, a toothed rotor is driven by the output shaft of the combustion engine to be tested. This toothed rotor, which is rotated, has teeth located at the periphery which cut the force lines of the magnetic field created by a coil. The cutting of the magnetic field by the teeth of the rotor and the gap between the lateral faces of the teeth and the adjacent metal parts give rise to eddy currents in the conducting material located on each side of the toothed rotor. These eddy currents generate a substantial amount of thermal energy in this material, which thus has to be cooled in order to remove the heat energy generated by the eddy currents.
To this end, in a known way, heat exchangers are placed on each side of the toothed rotor: these heat exchangers are cooled by a significant flow of cooling liquid.
Bearing in mind the significant braking effect due to the eddy currents, an eddy-current brake can be considered as being a device which converts mechanical braking energy into heat energy that has to be dissipated in these heat exchangers. These heat exchangers are subjected, when the combustion engine undergoing test changes speed and especially when it changes load, to very substantial temperature variations: hence, the material of the exchangers experiences temperature jumps from 20xc2x0 C. to 400xc2x0 C. as the speed of and load on the combustion engine increases. These temperature jumps give rise to thermal fatigue of the material of the exchangers, which leads to the appearance of cracks and to swift deterioration of the exchangers.
Numerous heat exchanger systems or principles for eddy-current brakes have been proposed, without yet being able to achieve sufficient endurance or reliability to allow for long-term use.
The mechanical, thermal and hydraulic stresses have led to a preferred type of exchanger in which the path of the cooling liquid involves significant pressure drops, which means that it is not possible to increase the rate of flow of cooling liquid through these exchangers.
In general, cooling water is used by way of preferred cooling liquid. This cooling water enters via an inlet located at the periphery of the exchanger and re-emerges via an outlet located toward the center of the exchanger.
A first object of the invention is to propose a new heat exchanger arrangement for eddy-current brakes, so that the rate of flow of cooling water can be increased and the temperature level experienced by the material of the exchanger can thus be decreased.
A second object of the invention is to allow long-term operation of an eddy-current brake device, to improve the availability of these devices and the length of time for which they may be used.
A third object of the invention is to reduce the pressure drops experienced by the cooling liquid as it circulates through the exchanger.
A fourth object of the invention is to propose an eddy-current brake device configuration in which the precision and quality with which engine torque is measured are increased, by virtue of a decrease in the torque induced by the cooling water circuits.
A fifth object of the invention is to propose a heat exchanger configuration that can replace heat exchangers for eddy-current brakes of the known type.
A subject of the invention is an eddy-current brake device, of the type comprising at least one heat exchanger for dissipating the thermal energy that is due to the eddy currents during braking, characterized in that the heat exchanger is designed to define a path for the cooling liquid, which path is predetermined in order to minimize the pressure drops and maximize the rate of flow of cooling liquid circulating through the exchanger, with a view to reducing the variations in working temperature of the heat exchanger.
The device according to the invention comprises two symmetric heat exchangers, of which the inlets, the circuits and the outlets for water are arranged symmetrically, so as to compensate for the forces due to the currents of cooling liquid and so as thus to minimize the corresponding residual torque.
According to other features of the invention:
the path of cooling liquid is predetermined so that the circulation of the cooling liquid is practically constantly in one and the same direction of winding,
the path of the cooling liquid may comprise several sections, for example of rectangular profile or of rectangular and semi-toroidal profile, connected together by connecting ducts; or, alternatively, the path of the cooling liquid may be shaped approximately in the form of a spiral,
the path of the cooling liquid may, in another embodiment, be produced at least partially by winding a tube of approximately constant cross section,
the circulation of liquid in each of the two exchangers is preferably approximately about the axis of rotation of said rotor, the directions of circulation in said two exchangers being the opposite of one another.